Oxygen delignification of high-yield kraft pulp part I: Structural properties of residual lignins

The chemistry of oxygen delignification of high-yield kraft pulp was studie d by analysis of residual lignin extracted from kraft and kraft-oxygen pulp s using the acid hydrolysis/dioxane extraction method. For reference pulps cooked to kappa numbers between 20 and 25, the content of free phenolic gro ups decreased to about 50% the original value upon oxygen delignification, while the content of carboxylic acid groups increased by 50-100%. For ligni ns isolated from high-yield kraft pulp and oxygen delignified high-yield kr aft pulp, it was shown that high-yield kraft pulping with polysulfide (PS) and anthraquinone (AQ) gives a residual lignin which is chemically differen t from that of kraft pulps cooked to lower kappa numbers. Lignin extracted from oxygen delignified high-yield PS/AQ kraft pulp was more similar to lig nins extracted from kraft pulps cooked to lower kappa numbers.